I am noob when it comes to camera specs, but judging by what you put in your post are you implying that... the most similar lens, to match with a gopro when its set to wide - would be the "Samyang NX 8mm"?

what would you personally use for shooting stills of tandems?

cheers for the detailed info, and for the fast reply! your help is very appreciated, i don't want to waste money on buying the wrong lens.

Samyang NX 8mm is closest to GoPro set to wide, it also has similar fisheye distortion, but i think this setup will be too wide for tandems or anything outside formation except some very creative shots from very close distance.

i would personally use Samsung NX 16mm and GoPro set to narrow (you should have a bit wider picture than these shooting with Canons + kit lens and video should have less distortion) or take Samsung NX 12-24 if you plan to use GoPro with narrow and also middle field of view.